The present invention is directed to Digital Light Processing (DLP) projection systems, and more particularly to a method and apparatus for synchronizing operation of an AC mercury lamp and DMD (Digital Micromirror Device) in a DLP projection System for 3D simulation.
Digital Light Processing (DLP) projection is based on the use of a source lamp, an illumination system, and a color splitting-recombining light engine. The optical function of the light engine is to split uniform illumination white light into Red/Green/Blue (RGB) channels, merging the three channels onto an imaging device or optical panel such as an LCD (Liquid Crystal Display) or DMD (Digital Micromirror Device), and then re-combining all three channels into a single illumination light beam that is projected onto a screen via a projection lens.
The DMD is an electromechanical device consisting of millions of microscopic mirrors that modulate light by independently flipping each mirror through a +−12 degree angle, between an “on” state for reflecting the source light to be viewed on screen and an “off” state for diverting the light to a light dump. Intermediate light intensities on screen are generated by toggling the DMDs according to a pulse width modulation (PWM) method, at frequencies above human perception in order to achieve time-averaged values. In this respect, the exact grey level produced depends on the dwell time of each “on” or “off” state.
PWM toggling of the DMDs must account for any time dependent changes in the source lamp brightness as the image is being rendered. Alternating Current (AC) mercury (Hg) lamps are one example of a light-source with very favourable properties, such as long life and high efficiency, but whose light output is not constant. These lamps typically have a step-like waveform consisting of a long, constant “plateau” level followed by a higher output-maintenance “pulse” that is used to improve lamp lifetime. The DMD “on” time must be inversely related to the “plateau” and “pulse” source lamp levels in order to provide a proper grey scale image on screen. This can represent a practical implementation challenge to the use of AC Hg lamps in DLP projection systems since each lamp model requires customized toggling patterns (“DMD bit sequences”), and a given lamp's AC waveform may even change over its lifetime, with a drifting ratio of pulse and plateau levels. Further, DMD speed limitations are such that it is not currently possible to correct AC lamp waveforms at the 90-120 Hz frame rates useful for 30 applications.
When used for 3D simulation, DLP projectors must be able to produce fixed intervals of darkness within each frame of video data, in addition to rendering an image. As indicated above, the DMD renders an image over a portion of the video frame period, and then spends the remaining time in the “off” state, which results in dark intervals between each rendered image. In 3D applications, this dark interval is used as a timing mask to hide the state transitions of the viewer's LCD eye shutters. If the dark interval is not present, any image on screen during the shutter transitions would be seen by both eyes, causing image artifacts. Inserting dark time requires that the image content be projected over a compressed time period, which can require unique bit sequences depending on how the lamp waveform scales relative to the image rendering period.
Accordingly, it is an objective of the present invention to address the complications and performance limitations set forth above of using pulsed lamp sources in DLP projectors used for 3D simulation.